1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication device, a wireless communication method, and a computer program, which are used for enabling intercommunication among a plurality of wireless stations.
More particularly, the present invention relates to a wireless communication system, a wireless communication device, a wireless communication method, and a computer program, which are used for measuring or specifying a geographical position of a wireless terminal in a wireless network. More specifically, the present invention relates to a wireless communication system, a wireless communication device, a wireless communication method, and a computer program, which are used for measuring or specifying a geographical position of a wireless terminal in a wireless network with a granularity that is finer than a cell size.
2. Description of Related Art
Configuring a LAN (Local Area Network) by connecting a plurality of computers enables sharing of information, such as files and data, and peripheral equipment, such as a printer, and also enables information exchange, such as transfer of e-mails or data contents.
Hitherto, a wired LAN connection using optical-fiber cables, coaxial cables, or twisted-pair cables has commonly been employed. In this case, line-laying is needed, so that it is difficult to easily build a network, and that cable drawing is troublesome. The wired-LAN connection is inconvenient because the moving range of equipment is limited by cable length even after the LAN is built. Accordingly, a wireless LAN attracts attention as a system for freeing users from the wiring of a related art wired LAN. This wireless LAN saves most of wiring cables in a working space of an office. Consequently, communication terminals, such as personal computers (PCs), can relatively easily be moved therein.
In recent years, with increase in operating speed and reduction in cost of a wireless LAN system, demands therefor have extremely increased. Recently, the introduction of a personal area network (PAN) is studied so as to build a small wireless network built among a plurality of electronics provided in a person's immediate environment.
Different wireless communication systems have been prescribed by utilizing frequency bands, such as the 2.4-GHz band or the 5-GHz band, the frequencies of which are available without permissions from competent authorities. An IEEE 802.11 wireless network system utilizes radio waves at frequencies of the 2.4-GHz band or the 5-GHz band and enables spatial information transmission at a speed ranging from several Mbps to several tens Mbps. In many wireless networks, wireless transmitting/receiving devices, which are enabled to perform effective communication within a “cell” coverage under a control of a wireless base station, operate and perform intercommunication among them. Generally, each cell has a diameter ranging from several tens meters to several hundreds meters.
Usually, one service area is provided with a plurality of wireless base stations. A wired network intercommunicates among these base stations. Each of the wireless base stations notifies wireless terminals, which are present in a cell, of the presence of a cell coverage area thereof by periodically broadcasting a radio frame, which is called a beacon frame (incidentally, a beacon frame is represented by an identification signal), to the wireless terminals. Thus, a wireless network is built. Additionally, this beacon frame may include information for performing a communication control operation, such as an operation of assigning a time slot to a specific terminal.
FIG. 1 illustrates a multiple access method using CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) technique, which is employed in an IEEE 802.11 system.
This figure illustrates a case that a base station communicates with two terminals A and B. An example illustrated in this figure is adapted so that the base station performs unicast communication with each of the terminals, and that when a data frame is accurately received by a receiving portion, the receiving portion informs a transmitting portion of success of the communication by returning an ACK frame thereto.
FIG. 2 illustrates a format of a data frame according to the IEEE 802.11 standard. FIG. 3 shows a format of the ACK frame. In the format of each of these frames, a MAC (Machine Access Control) header has a Duration/ID field for which a value updating a time required until the completion of a sequence of communication operations is substituted. When a terminal, which currently transmits no data, receives this field, this terminal is enabled to predict a time when this terminal becomes able to transmit data.
According to a CSMA/CA method, radio waves are asynchronously transmitted from the base station and the terminals. Thus, to avoid a collision of frames and to realize priorities of transmissions of data frames according to the kinds of the data frames, a contention period, a backoff period, and periods each called an IFS (Inter-Frame Space) interval, during which the transmissions are inhibited, are set between the data frames. Incidentally, in FIG. 1, “DIFS” is an abbreviation of “DCF IFS” (additionally, “DCF” is an abbreviation of “Distributed Coordination Function” described in the IEEE 802.11 specification), and “SIFS” is an abbreviation of “Short IFS” also described therein. The values of the SIFS interval and the DIFS interval are 16 microseconds and 34 microseconds as practical values of the IFS interval according to the IEEE 802.11 standards, respectively.
FIG. 4 illustrates a multiple access method using a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) technique. According to this method, transmission and reception operations in the base station and all the terminals accessing thereto under the control of the base station, are performed in synchronization with one another. In an example illustrated in this figure, a frame period of a frame repeated in a predetermined cycle is divided into a plurality of slot periods that are assigned to periods during which the transmission and reception operations are performed by the base station and the terminals. Because the duration of each of the slot periods is constant, a time period unused for data communication may occur in a case that a time required to transmit data is shorter than the slot period. Additionally, transmission inhibition periods for preventing occurrences of collisions of transmission slots due to the difference in distance between the base station and each of the terminals and in transmission timing therebetween, that is, guard periods are set in the frame period.
Meanwhile, in a wireless network, each of wireless terminals usually can geographically move. To enable a wireless terminal to continuously make communication even when the wireless terminal goes out of a cell covered by a certain base station, it is necessary to perform an operation of successively changing a base station, to which the wireless terminal is connected, with the movement of the wireless terminal. Generally, such an operation of changing the base station is called a “handover”.
To perform a handover, it is necessary that a wireless terminal knows the presence of a base station by receiving a beacon frame transmitted from the base station. The wireless terminal selects a base station, which can provide better communication thereto, by utilizing the electric field strength (indicated by RSSI (Received Signal Strength Intensity) signals) of the beacon or utilizing, during the wireless terminal is connected to a base station, the RSSI of a data communication frame so as to be connected to the base station and as to make judgment on the base station. In such a system, during the terminal is connected to the base station, the position of the terminal can be known with the granularity that is equal to the size of the cell covered by the base station.
The terminal connected to the base station can exchange information with various servers through a wired network connecting between the base stations. In this case, a user can enjoy services provided by the data communication.
Related art wireless network techniques do not provide a mechanism for measuring the geographical position of a terminal. Because only rough conjecture thereof employing the size of the cell, which is covered by the base station, as the granularity can be performed, in a case that the coverage area thereof is large, and that a plurality of cells overlap with one another, a problem may occur. It is impossible to discriminate among a plurality of terminals connected to the same base station.
In a case that a problem occurs when the position of a terminal is specified with the granularity that is equal to the size of the wireless cell, it is considered to adopt a more accurate approach of combining ultrasonic-based or infrared-based techniques of measuring the position of a wireless terminal with a device for data communication. However, it is necessary to mount two systems, that is, a data communication system and a position measuring system in a wireless communication device. This results in increase in size and cost of the apparatus.
It is also considered to adopt another approach of measuring the position of a wireless terminal by causing a base station to first transmit a data frame and to measure the distance therefrom to the wireless terminal according to a time from the transmission of the data frame to reception of an ACK frame from the wireless terminal. However, it is necessary for realizing the distance measuring accuracy of, for example, 1 meter to complete the generation and transmission of an ACK frame in a time of less than 10 nanoseconds. Consequently, a complex and expensive system is needed.
Recently, a cellular telephone system combined with a GPS (Global Positioning System) so as to provide means for detecting a position of a terminal has emerged. This is a technology of a US company, QUALCOMM, Incorporated that is called gpsOne (incidentally, gpsOne is a trademark of QUALCOMM, Incorporated). According to this technology, a wireless terminal having received a positioning signal from a GPS satellite performs signal processing in cooperation with the base station. Usually, each cell coverage area in a cellular telephone network has a diameter of several hundreds meters to several kilometers. According to this system, the geographical position of the terminal can be identified with the accuracy of about several meters.
Although such a GPS can identify the position of a terminal with the precision of several meters, this function cannot be utilized with high precision in an environment that cannot get radio waves from the GPS satellite. Thus, a constraint is imposed to the indoor utilization of radio waves sent from a GPS satellite. A circuit for processing GPS signals is also needed. Consequently, the whole system is complicated.
For instance, the specification of Japanese Patent Application No. 3-113927 discloses a system for identifying a position of a transmitter. However, it is necessary to provide a time reference station in a measuring region. Additionally, this system has no functions for data communication.
Japanese Patent Laid-Open No. 5-60854 discloses a system for measuring a position of a transmitter. This system aims at realizing a measurement of the position without employing a reference transmitter and a complex circuit configuration. However, this system has encountered a problem in a case that a time delay between a reference station and a computer for computing the position affects the measuring accuracy.
According to the related art wireless LAN techniques, an accurate position of a terminal cannot be measured. The related art has drawbacks in that it is necessary for realizing the accurate measurement of the position of the terminal to achieve the synchronization among the wireless base stations and to realize the highly agreement of clock accuracy among them, that the scale of the circuit increases, and that the cost of managing the base station increases. Thus, the related art is not suitable for measuring the position of a terminal in a wireless LAN system in which many base stations are disposed.